1. Field of the Invention
The present invention relates to a method for producing a metallic oxide-hydrogen secondary battery which uses as negative electrodes hydrogen-absorbing electrodes capable of reversibly absorbing and desorbing hydrogen.
2. Description of the Prior Art
In general, hydrogen-absorbing electrodes which use, as components, alloys capable of reversibly absorbing and desorbing hydrogen are produced by the following method:
First, various kinds of metals are weighed in accordance with desired compositions of hydrogen-absorbing alloys to be produced. A mixture of these metals are dissolved by using an arc melting furnace to produce hydrogen-absorbing alloys having desired compositions. Then, these hydrogen-absorbing alloys are ground to powders each having a size of 300 mesh or less. These powders are kneaded with a binding agent so as to be uniform paste. Electrode bases such as foaming porous metals and panting metals are pressure-charged or coated with this paste, followed by drying to produce hydrogen-absorbing electrodes which are to be used as negative electrodes.
These negative electrodes, positive electrodes such as nickel positive electrodes, and separators are disposed in a battery case, thereby forming a metallic oxide-hydrogen secondary battery. When this battery is charged and discharged, a required amount of alkaline electrolyte is uniformly provided in each cell chamber in the battery case. As an alkaline electrolyte used in the metallic oxide-hydrogen secondary battery, an aqueous KOH solution is generally used. Moreover, as a battery case, in view of strong alkalinity of the electrolyte, moldability, and cost, various kinds of plastics such as an acrylonitrile-styrene (AS) copolymer are generally used.
The above-mentioned metallic oxide-hydrogen secondary battery has an electro motive force of about 1.2 V and is usually connected in series except when it is used for a special purpose. In order to obtain a desired voltage, a plurality of cells are connected with each other. When this battery is used as a portable power source, it is preferred that this battery is formed into a monoblock multi-cell type battery having an output voltage of 6 V or 12 V.
The battery case is divided into a plurality of cell chambers with inter cell partitions and each cell chamber is designed to have predetermined capacity. However, it is difficult to make the capacity of each cell chamber completely uniform. Moreover, thickness and porosity of generating elements including the negative electrodes, the positive electrodes, and the separators differ from cell to cell. Accordingly, even though an equal amount or an equal level of electrolyte is poured into each cell chamber, respective liquid surfaces are different due to the decrease in the amount of electrolyte and the permeation of the electrolyte into the generating elements, which are caused by the repetition of charge and discharge cycles. As a result, discharge capacity of the respective cells becomes different, and the battery life is decreased.
Furthermore, in the metallic oxide-hydrogen secondary battery, the capacity of the negative electrodes is greater than that of the positive electrodes. During overcharge, oxygen gas is generated at the positive electrodes and this oxygen gas is absorbed at the negative electrodes, thereby maintaining internal pressure of the battery at a constant value. The reaction in which oxygen gas is absorbed at the negative electrodes is greatly influenced by the amount of the electrolyte. Therefore, it is required that the amount of the electrolyte be regulated. When the amount of the electrolyte is small, the internal resistance is increased, leading to a decrease in the discharge capacity. In order to avoid this, a minimum amount of the electrolyte required for a normal discharge reaction should be supplied. In contrast, when the amount of the electrolyte is large, oxygen gas is generated at the positive electrodes without absorbing at the positive electrodes, and then the amount of the electrolyte is decreased. As a result, the internal resistance is increased and the discharge capacity is decreased.
A safety value is generally used for the purpose of regulating pressure in each cell. When a safety valve working at high pressure is used, because of the low resistivity of the case with respect to pressure and the low strength of a contact portion between the case and a lid, the contact portion is likely to be damaged, thereby causing the leakage of the electrolyte. As a result, battery capacity is decreased. For safety, it is desired that a safety valve working at high pressure is not used.
In a metallic oxide-hydrogen secondary battery using a case dividing into a plurality of cell chambers, each having different capacity, the amount of the electrolyte supplied in each cell chamber is regulated and the respective electrolyte levels are adjusted. That is, the capacity of each cell chamber is measured, and the electrolyte is poured into each cell chamber in accordance with a measured value. However, according to this method, it takes a long time to pour the electrolyte, and it is difficult to regulate the amount of the electrolyte to be supplied since the electrolyte is supplied in accordance with the measured value. Moreover, when the charge and discharge cycles are repeated, the amount of the electrolyte which is absorbed at the elements are not constant depending on the cells, so that there are some cells in which the amount of the electrolyte deviates from the optimum value.